As shown in FIG. 1, there is known a piston-type hydraulic pump-motor in which a cylinder block 3 secured to a shaft 2 is disposed inside a casing 1 to be rotatable, plurality of cylinder chambers 6 are formed into which pistons 5 are respectively fitted in a plurality of cylinder bores 4 of the cylinder block 3. The respective pistons 5 are disposed to be slidable in respective axial directions thereof in accordance with the rotation of the cylinder block 3 through slidable abutment of outer end portions of the pistons 5 against a swash plate 8 through piston shoes 7. And as shown in FIG. 2, the cylinder chambers 6 are communicated with high and low pressure ports 10 and 11 formed in a valve plate 9 so as to alternately at every 180.degree. rotation.
In such a hydraulic pump-motor, as shown in FIG. 2, ports 12 open to the cylinder chambers 6 alternately communicated with the high pressure ports 10 and the low pressure port 11 at top and bottom dead points of the valve plate 9 to thereby switch the operation "from drain to suction" and "from suction to drain".
Fine grooves 10a and 11a are formed with the high and low pressure ports 10 and 11 so as not to cause a rapid pressure change at the time of this switching operation.
That is, if the port 12 is suddenly communicated with the high or low pressure port 10 or 11, the pressure in the cylinder chamber 6 rapidly changes, thus causing hydraulic pressure pulsation or large noise. In order to prevent the pressure in the cylinder chamber 6 from rapidly changing, the fine grooves 10a and 11a are formed with the high and low pressure ports 10 and 11 so that the ports 12 are gradually open thereto through the fine grooves 10a and 11a.
However, since the shapes and sizes of the fine grooves 10a and 11a are made constant and the valve plate 9 is not moved, the port 12 always assumes a constant open position. Accordingly, it is difficult to always achieve most suitable operational characteristics at the time of changing the rotating speed of the cylinder block 3 or the maximum pressure in the cylinder chamber 6 at which the hydraulic pressure pulsation or noise will be caused.